The plasma synthesis of nanopowders has attracted increasing attention over the past few years. Numerous processes have been developed for preparing metal, alloy and ceramic-based nanopowders using a wide range of technologies comprising inductively coupled plasma discharge, arc discharge, electro-explosion, self propagating high temperature synthesis, combustion synthesis, electric discharge, spray pyrolysis, sol-gel, and mechanical attrition.
High temperature plasma processing routes are based on the concept of heating the reactant precursors (in solid, liquid or vapor/gaseous form), to relatively high temperatures, followed by rapid cooling of the reaction products through their mixing with a cold gas stream as in the “high intensity turbulent quench technique” or through their contacting with a cold surface on which the nanoparticles form and deposit. However, a drawback of the use of traditional “cold-surface” condensation techniques is that the nature and the temperature of the condensation surface changes with the build-up of the condensed nanopowder layer. The use of a “highly turbulent gas quench zone” has been previously described by Boulos et al. in U.S. 20050217421 and U.S. 20030143153 filed on Mar. 25, 2005 and Dec. 6, 2002 respectively. The use of a renewable gaseous cold front has been previously described by Boulos et al. in U.S. 20070029291 as filed on Jan. 27, 2006. A common objective to all of these processes is the desire to closely control the particle morphology, the particle crystallinity, the particle size distribution, and the agglomeration of the powders obtained.
U.S. Pat. No. 6,379,419 issued to Celik et al. on Apr. 30, 2002 discloses a transferred arc thermal plasma based vapor condensation method for the production of fine and ultra fine powders. The method calls upon a conventional two-step condensation (i.e. cooling) procedure including an indirect cooling step and a direct cooling step. The indirect cooling step involves a cooling surface whereas the direct cooling step involves the injection of a cooling fluid, liquid or gaseous, directly onto the vapor. The vapor laden stream of hot gas is first subjected to an indirect cooling or heating section providing for a control over the growth and crystallization of the particles, followed by a direct cooling section wherein the cooling is performed by injecting a fluid, liquid or gaseous, directly onto the vapor and aerosol laden stream. The use of a cooling surface suffers from the drawback of particle build-up on the condensation surface.
It has been shown theoretically that by controlling the initial vapor concentration and temperature, residence time of particle nucleation and growth, and cooling profile, one may have some control on the particle size distribution and crystallinity. This is shown by Okuyama et al. in AlChE Journal, 1986, 32 (12), 2010-2019 and Girshick et al. in Plasma Chem. and Plasma Processing, 1989, 9 (3), 355-369. However, these references remain silent as to an efficient method for producing nanopowders of well defined particle size distribution and morphology.
The present invention refers to a number of documents, the content of which is herein incorporated by reference in their entirety.